This invention relates generally to fittings for joining articles, and more particularly, to a joint configuration for forming lap joints to a thermoplastic pipe by thermal bonding using an electrical heating element.
Thermoplastic polymers are used widely to manufacture articles such as pipes and pipe fittings which require good corrosion and chemical resistance, low weight and good fabricability. The joining of articles made of thermoplastic material may be accomplished by mechanical means such as threaded connections and flanges, by chemical means such as solvent bonding, or by thermal means such as fusing thermoplastic material at the location where the articles to be bonded meet. As an example, in a conventional approach a pipe may be thermally bonded to a fitting by providing a diametrically enlarged female portion on the fitting, heating the inside of the enlarged portion and the outside of the end of the pipe to fuse the thermoplastic on each, and forcing the pipe end into the enlarged portion while the thermoplastic is in a fused, somewhat fluid state. Upon cooling, the fused thermoplastic materials join and bond the articles together.
When a joint is formed by thermal bonding, it is desirable that the areas joined be relatively extensive to provide a large force-bearing area for high joint strength and a continuous layer around the entire circumference of the joint to seal the liquid within the pipes, and it is further desirable that the joint be sound and without internal defects such as air bubbles which might reduce the fracture strength of the joint by providing a fracture path for cracks. However, when articles are joined by the conventional thermal bonding approach described above, the joint may not be satisfactory, as some of the fused thermoplastic may be forced into the inside of the fitting and pipe to impede fluid flow, and because there may be leakage paths through the joint resulting from the sticking of the fused thermoplastic to the tooling of the heating unit.
In an approach to improving the bonding in the joint, the enlarged portion is provided sufficiently oversize that a heating coil may be placed between the outside diameter of the pipe and the inside of the enlarged portion of the fitting and heated by the passage of an electrical current to fuse the thermoplastic lying around and adjacent the heating coil with the pipe already in place within the enlarged portion. In thermal bonded joints formed by this approach, as the thermoplastic fuses it becomes free to flow within the joint area, and as a result may simply flow away from some portions of the intended joint without achieving a desirably extensive, continuous, and sound bond. While such thermal-bonding techniques are often satisfactory for bonding pipes for use in unpressurized systems such as drain lines, where the fluid flowing through the pipes is unpressurized and does not exert significant mechanical forces on the joints. However, where the fluid flowing through the pipe is to be pressurized, conventional thermal bonding techniques are often unsatisfactory because the bond is insufficiently strong and does not adequately seal the joint.
Yet another approach to improving the joint formed in thermal bonding has sought to ensure an extensive, continuous bond by winding the electrical heating coil directly on the rigid core used to form the fitting, thereby achieving a close, fully circumferential contact between the fitting and the fusible thermoplastic of the core. This approach has the inherent disadvantage that the inner diameter of the core must be made sufficiently large to accommodate an inserted pipe having a maximum diameter as allowed by the pipe specification, with the result that another inserted pipe having the smallest diameter allowed by the pipe specification will fit into the core very loosely with a possibly large gap which may not be fully eliminated by the thermal bonding, leaving unbonded portions and air bubbles in the joint. And, the expanding thermoplastic is still free to flow out of the joint causing a poor bond. Moreover, this approach requires hand layup of the cores and therefore is costly.
Accordingly, there has been a need for a means to form extensive, continuous and sound joints between thermoplastic articles by thermal bonding in an economical fashion. The present invention fulfills this need.